Polybenzimidazoles (PBI) are a known class of compounds, see the discussion below. Also see: Powers, E. J. and Serad, G. A., History and Development of Polybenzimidazole, Symposium on the History of High Performance Polymers, American Chemical Society (1986). PBI may be made by either a melt/solid polymerization or a solution polymerization. To date, the most prominent commercial process for the manufacture of PBI has been the melt/solid polymerization.
The melt/solid polymerization generally is performed in two steps. See, for example, U.S. Pat. Nos. Re. 26,065; 3,433,772; 3,509,108; 3,551,389; 3,655,632; 4,312,976; 4,717,764; & 7,696,302 each is incorporated herein by reference. The first step is the reaction of the monomers, and the second step increases the molecular weight of the polymer. The polymer typically requires an inherent viscosity (IV), measured in sulfuric acid, of greater than 0.4 dl/g (typically 0.50-1.20). This polymer, or resin, may be subsequently formed into, among other things, fibers and films. The process for forming fibers and films requires the production of a dope (about 25% by weight polymer in solvent for fiber spinning operations, about 15% for film casting operations, about 10% for hand casting of films), and extrusion of the dope into either the fiber or film with the evolution of the solvent. While this process has produced a PBI that has met with great commercial success, the cost of producing the polymer has been limiting in the wide industrial acceptance of the polymer.
Solution polymerization is also known, see Powers and Serad, mentioned above, page 2. However, the solution polymerization method has been nothing more than an academic novelty. A significant problem is that solution polymerization has been conducted at low monomer concentrations. Another problem is that as the monomer concentration in the solution increases, the polymer's IV decreases. See for example, Mader, J. et al., Polybenzimidazole/Acid Complexes as High-Temperature Membranes, Adv Polym Sci (2008) 216: 63-124, page 103, FIG. 14 (reproduced here as FIG. 1). Because the solution method has not produced polymer with a sufficiently high IV or a solution with a sufficiently high solids content, this method has not been exploited commercially.
There is a need for a method of making a polybenzimidazole having a sufficiently high IV and a high solids content in solution.